In mobile networks, it is known to use radio access technologies such as GSM (Global System for Mobile Communications), WCDMA (Wideband Code Division Multiple Access) or 3GPP LTE (3rd Generation Partnership Project Long Term Evolution). In these radio access technologies, the mobile network is organized in cells, each cell being served by a base station or a sector of a base station. The different radio access technologies typically operate in different carrier frequency bands, and frequency distributions among operators and/or among the different radio access technologies are typically controlled by the local authorities. A certain radio access technology may also utilize multiple carrier frequency bands at the same time to increase network capacity. Such mobile networks may also be referred to as multi-carrier mobile networks. If only a single frequency band is used, the mobile network may be referred to as a single-carrier mobile network.
The cellular structure of the above-mentioned mobile networks allows for supporting mobility of a user equipment (UE). The UE can connect to the mobile network in a certain cell and move to other cells of the mobile network without interruption of service. Mobility is achieved by performing handovers of the UE between the cells. At a given location, there may be an overlap of cells belonging to the same or to different mobile networks. A handover may then be performed between such overlapping cells for which a neighbor relation is configured. The handover decision is typically based on radio measurements and certain policies. Depending on the particular situation, different types of handover can be distinguished: intra-frequency handovers, inter-frequency handovers, and inter radio access technology (IRAT) handovers. Intra-frequency handover take place between cells using the same frequency band. Inter-frequency handovers take place between cells using different frequency bands of the same radio access technology, e.g., as supported in WCDMA or LTE. IRAT-handovers take place between cells using different types of radio access technology, e.g., from a GSM cell to a WCDMA cell or to an LTE cell.
The cell structure of a mobile network may be determined by planning tools, e.g., based on mostly geographical information and expected radio propagation characteristics. The goal of cell planning typically is to provide a full geographical coverage and appropriate cell relations in the service area. In many case, the initially planned cell structure is determined separately for each type of radio access technology, and inter-radio access technology relations are added later in order to make handover between the different radio access technologies possible. When geographical coverage or capacity of the mobile network is not enough to support increasing traffic, cells may be added to the mobile network. In a multi-carrier mobile network, cells may be first added in the a first frequency band. If no further increase of the network capacity is possible within the first frequency band, cells may be added in a further frequency band. The resulting cell structure in a mobile network using multiple radio access technologies and/or multiple frequency bands can be quite complex.
The neighbor relations between cells are usually determined by geometrical considerations during planning of the cell structure. When new cells are activated, the neighbor relations can be optimized on the basis of radio measurements performed by UEs and handover related statistical data. For example, the UEs can be requested to measure and report the signal strength of the new cells. If a sufficiently strong signal is measured in a given serving cell, a neighbor relation between the serving cell and the new cell is established. Further, in the case of a failure rate of handovers between two cells or in the case of a low rate of handovers between two cells, the existing neighbor relation between the two cells can be removed.
A mobile network may utilize omni-cell and 2-6 sector cell structures: Large areas with a typically small number of UEs to be served may be covered by a large omni-cell. If there is a high density of UEs to be served, smaller and more densely arranged sector cells may be used to provide a higher capacity. According to some concepts, a base station may provide multiple sector cells which may be aggregated to an omni-base station configuration, thereby allowing for energy saving during low traffic periods. The capacity of a cell can be regarded as more or less constant. Accordingly, a higher cell density in a certain area allows for serving more UEs at the same time.
Accordingly, in a process of expanding a mobile network, cells are typically added to the existing cell structure, e.g., in areas with insufficient geographical coverage or insufficient capacity, i.e., if the traffic load approaches the capacity of the existing cells. If an expansion in the existing frequency band is no longer possible, a new frequency bands may need to be used. This in turn requires investments, e.g., for licensing the new frequency band or new radio equipment. On the other hand, fully exploiting the new frequency band is typically not possible because the traffic increase in the traffic load is gradual. Accordingly, network expansion may leave capacity unexploited for relatively long time.
Accordingly, there is a need for techniques which allow for efficiently managing the cell structure of a mobile network.